On 1 October 1935 Wemyss was promoted colonel[6] and appointed an Assistant Adjutant General at the War Office in London.[5] He was a student at the Imperial Defence College between 18 January 1938 and January 1939. On completing the course, he was promoted to major general[7] and appointed as the Director of Mobilisation at the War Office on 9 January 1939.[5] On 19 February 1940 he transferred to the post of Deputy Adjutant General to the Forces at the War Office.[5]
On 10 June 1940, nine months after the outbreak of the Second World War, Wemyss replaced General Robert Gordon-Finlayson as Adjutant-General to the Forces.[5] He was promoted Acting lieutenant general on assuming the role.[8] The increasing importance of the role of the United States in the Second World War led to United Kingdom government establishing a military mission to Washington, D.C. On 3 June 1941 Wemyss was appointed the Head of the British Army Mission to Washington.[5] In January 1942, Prime Minister Winston Churchill decided to replace Wemyss in Washington with General Sir John Dill[9] so Wemyss returned to the United Kingdom. Wemyss was appointed Military Secretary to the Secretary of State for War on 16 June 1942.[5] He was to remain in this key post for the rest of the war. He was promoted general on 15 October 1945, by which time the war was over, and retired from the army, after having served for almost exactly thirty-six years of service, on 23 November 1946.[5][10]
